Aerobic exercise training induces angiogenesis. The mechanisms and physiological signals which trigger exercise-induced angiogenesis are poorly understood. The objective of this study is to elucidate the growth factors responsible for exercise-induced angiogenesis and the pathways through which angiogenesis is initiated. And physiological variable which changes as a result of exercise could potentially trigger angiogenesis. Among the potential initiating signals for exercise-induced angiogenesis are exercise intensity, tissue hypoxia, tissue acid-base status, increases in muscle blood flow, increases in muscle perfusion pressure, nitric oxide, and prostacyclin. In diseases such as renal failure and certain cardiopulmonary diseases, a reduction in the capillary to muscle fiber ratio has been observed. By understanding the mechanisms which trigger exercise-induced angiogenesis, the capillary to muscle fiber ratio could be improved in these patients. Improvements in the capillary to muscle fiber ratio could result in improvements in the conductance of O2 and improve the condition of these patients.